I drive an old Daihatsu Charade 1993. Just 2 days ago, I heard a bubbling sound from the coolant when I turned off the machine. I didn't really care much until yesterday when the car got hot abnormally (3/4 of the gauge, usually 1/4) while idling, though the heat did lessen when the car moving, but it does imply the car has a trouble. Looking back, I guess it has something to do with the bubbling noise I had heard.

When I checked the coolant container I saw the liquid is still at just below max level which is fine, no leaking on the ground as well.

I drove again this morning, the car didn't get hot and I didn't hear the bubbling anymore, so I thought there wasn't any severe problem (I might be too optimistic). However, that symptom came back again just now when I drove for around 10 minutes. This time I noticed the coolant level got even higher, I still don't have any idea why, but surely it wouldn't be the head gasket thingy since I saw no leaking at all.

Now, as I did some research, there is a tendency saying this is a problem with the radiator cap being melt down or not being pressurized enough, but I'm not sure if it exactly is the case.

I think it'd be better to ask on here before bringing my car to the mechanic so I know the scope of the problem.

1 Answer
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I suspect you have a leaking head gasket, you don't have to see the coolant leak because it can leak into the combustion chamber and exit your vehicle in the form of vapor from the tail pipe. It can also leak into the oil so make sure to check the oil for contamination.

It could also be as simple as a bad radiator cap, you can test them with specialized equipment but they are so cheap I'd just replace it and see if the problem goes away. The job of the radiator cap is to allow the coolant system to pressurize to raise the boiling point. If the cap doesn't' allow the system to pressurize or reduces the amount of pressurization it can allow the coolant to boil and could be the bubbles you are seeing.

There is also an easy way to check the head gasket. They make a combustion gas leak detector that detects CO2 in the coolant indicating a leaking head gasket.

I agree it sounds like a head gasket leak. Note that there are many modes of failure for a head gasket, and it's likely that the ONLY direction of the leak will be exhaust gasses into the coolant. This will be the case if the gasket is physically intact and maintaining a seal except under extremely high pressure at the moment of combustion.
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R..May 20 '13 at 19:23

By the way, if using a test kit like the one Larry recommended, look for any, even the slightest, change in the color of the fluid. On a vehicle I tested which turned out to have a bad HG, the transition was only from blue to slightly teal, nowhere near yellow or even fully green. If you know what to look for, you can probably just observe the bubbling at the radiator cap and skip the test kit, but it can be hard to distinguish from just having air in the cooling system (which you'll have if the level is low).
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R..May 20 '13 at 19:28

Larry and R, thanks for the advice. I just wonder, if it was a head gasket problem then why would the coolant liquid level remain unchanged? If it leaks (to anywhere), I should see the liquid level went down shouldn't I? Anyway, I think I should bring it to the mechanics to check the head gasket, I don't want to see my car die this way.
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TrielmMay 21 '13 at 14:43

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@Trisism The head gasket can be seeping. The combustion chamber produces a much higher pressure than the cooling system, so you may have combustion gases leaking into the coolant but no coolant leaking into the combustion chamber, hence no loss of coolant.
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Move more comments link to top♦May 21 '13 at 17:18